1. Field of the Invention
The present invention relates to cleaning devices for downhole tools. In particular, but not exclusively, the invention relates to scraper devices having an improved bearing arrangement, the scraper devices being connectable within a drill string.
2. Brief Discussion of the Related Art
During oil and gas exploration and extraction, a drill string is rotated to form a well bore. The bore is often lined using a casing which generally comprises a number of steel pipes that are connected together as they are run into the bore. A number of different devices may be attached within the drill string, such as cleaning tools, collecting tools and impellers.
When rotating the drill string, it is often undesirable for the attached device to be rotated with the drill string. In some cases, the device may be in contact with the internal diameter of the casing. Rotation of the device relative to the casing may therefore result in wear to the device and/or to the internal diameter of the casing.
In order to avoid unnecessary wear, the device may be rotatably mounted relative to the drill string. This allows the drill string to rotate but the device does not rotate with the drill string.
One such type of device is a scraper tool which is used to clean the internal diameter of the casing in a well during longitudinal movement of the drill string. Typically, the scraper tool comprises a cylindrical body and a number of scraping blades extending from the cylindrical body. The scraping blades contact the casing to allow cleaning of the internal surface of the casing during vertical movement of the drill string. The cylindrical body is rotatably mounted relative to the drill string so that, if the drill string is rotated, the cylindrical body and the scraping blades do not. This arrangement avoids unnecessary wear to the scraping blades and/or to the internal surface of the casing as the scraping blades are not forced to rotate with respect to the casing.
However, one disadvantage inherent in the scraper tool being rotatably mounted relative to the drill string is that, upon rotation of the drill string, friction may occur at the interface of rotating (drill string) and non-rotating (cylindrical body) parts. This friction can result in damage to the device or downhole tool. For instance, galling can occur when parts rotate relative to each other. Friction occurs between surface asperities, which causes heat, and the asperities may weld together. Further rotational displacement of one part relative to another causes these micro-welds to break, which makes the interface surface even rougher, resulting in greater friction between the parts.
In extreme cases, such as at excessive speeds of rotation, the heat generated by the friction during rotation may cause the rotating and non-rotating parts to weld together at a macro level. The parts are typically formed from steel. Although steel typically has a melting temperature of over 1500° C., galling can occur at much lower temperatures, such as 800° C. The fusing of parts necessitates the carrying out of repairs and/or the replacement of the tool, which is expensive and time-consuming. Conventionally, to avoid these problems, the speed of rotation of the drill string is kept below a maximum value, even though higher drilling speeds would reduce the drilling time involved.